The present invention concerns linear actuators, and more particularly linear actuators for use with rotating shaft assemblies.
Linear actuators provide a linear range of motion and force useful in a number of applications. For example, linear actuators are often used to position and/or clamp work pieces in machine tools. Certain applications require the linear actuator to accommodate a rotatable shaft. For example, some lathe machine tools attach a chuck to a rotatable shaft actuated by a linear actuator. Using the linear actuator and chuck, a work piece is positioned and clamped in the lathe machine tool while it is rotated during the machining process. The rotation of the shaft, however, presents challenges to the design and operation of the linear actuator.
Linear actuators are typically operated using either mechanical systems (stepper motors, gears, springs, etc.) or pressurized fluid systems (gas or liquid). Many mechanical systems cannot be adapted for use with rotating shaft assemblies. Those systems which can be adapted for rotating shaft assemblies are typically overly complex and expensive. Additionally, mechanical systems usually have limited, if any, capability to adjust and apply different amounts of force for different applications.
Pressurized fluid systems provide their own set of challenges for applications involving rotating shaft assemblies. Conventional pressurized fluid systems rely on a constant delivery of pressurized fluid to the rotating components to maintain the pressure required to actuate the rotating shaft components. This delivery is generally accomplished using a rotary union, or similar structure, in which the pressurized fluid is supplied to a relatively fixed structure that surrounds a portion of a rotating structure. The rotary union delivers the pressurized fluid through passages that are isolated by a relatively small gap between the outer surface of the rotating structure and the adjacent inner surface of the stationary structure. This gap in conventional rotary unions results in fluid leakage and pressure loss. Fluid shear from passing through the small clearance generates excessive heat. The small clearance requires constant lubrication and is subject to fluid contamination in the rotary union which can lead to device failure. The need for lubrication and the pressure loss are the reasons conventional units require a constant delivery of pressurized fluid. Placing pressure seals between the stationary and rotating structures to prevent leakage and avoid contamination still results in heat build-up as well as excessive wear on the components due to the constant contact between the rotating and fixed structures.
A need exists for a linear actuator for use with rotating shaft assemblies that has the ability to deliver and maintain fluid pressure in selectable amounts without requiring the fluid pressure source to remain in contact with the rotating shaft while it is rotating.